Make "kyua report-html" less verbose on stdout

Make "kyua report-html" less verbose on stdout

"kyua report-html" command in CI generates more than two pages of output
to stdout, which is nothing but which HTML pages Kyua generated, e.g.:

  Generating kyua_html/context.html
  Generating kyua_html/lib_dns_tests_acl_test_main.html
  ...
  Generating kyua_html/lib_ns_tests_query_test_main.html
  Generating kyua_html/report.css
  Generating kyua_html/index.html

This is seldomly useful and requires the user to scroll three pages
upwards to get to unit test results.